Several diseases and conditions of the posterior segment of the eye threaten vision. Age related macular degeneration (ARMD), choroidal neovascularization (CNV), retinopathies (e.g., diabetic retinopathy, vitreoretinopathy), retinitis (e.g., cytomegalovirus (CMV) retinitis), uveitis, macular edema, and glaucoma are several examples.
Age related macular degeneration (ARMD) is the leading cause of blindness in the elderly. ARMD attacks the center region of the retina (i.e., macula), responsible for detailed vision and damages it, making reading, driving, recognizing faces and other detailed tasks difficult or impossible. Current estimates reveal that approximately forty percent of the population over age 75, and approximately twenty percent of the population over age 60, suffer from some degree of macular degeneration. “Wet” or exudative ARMD is the type of ARMD that most often causes blindness. In wet ARMD, newly formed choroidal blood vessels (choroidal neovascularization (CNV)) leak fluid and cause progressive damage to the retina. About 200,000 new cases of Wet ARMD occur each year in the United States alone.
Brachytherapy is treatment of a region by placing radioactive isotopes in, on, or near it. Both malignant and benign conditions are successfully treated with brachytherapy. Lesion location dictates treatment technique. For the treatment of tumors or tumor beds in the breast, tongue, abdomen, or muscle capsules, catheters are inserted into the tissue (interstitial application). Radiation may be delivered by inserting strands of radioactive seeds into these catheters for a predetermined amount of time. Permanent implants are also possible. For example, in the treatment of prostate cancer, radioactive seeds are placed directly into the prostate where they remain indefinitely. Restenosis of coronary arteries after stent implantation, a non-malignant condition, has been successfully treated by placing a catheter into the coronary artery, then inserting a radioactive source into the catheter and holding it there for a predetermined time in order to deliver a sufficient dose to the vessel wall. Beta emitters, such as phosphorus 32 (P-32) and strontium 90 (Sr-90), and gamma emitters, such as iridium 192 (Ir-192), have been used. The Collaborative Ocular Melanoma Study (COMS), a multicenter randomized trial sponsored by the National Eye Institute and the National Cancer Institute demonstrated the utility of brachytherapy for the treatment of ocular cancers and/or tumors. The technique employs an invasive surgical procedure to allow placement of a surface applicator (called an episcleral plaque) that is applied extraocullarly by suturing it to the sclera. The gold plaque contains an inner mold into which radioactive iodine 125 (I-125) seeds are inserted. The gold plaque serves to shield the tissues external to the eye while exposing the sclera, choroid, choroidal melanoma, and overlying retina to radiation. The plaque remains fixed for a few days to one week in order to deliver approximately 85 Gy to the tumor apex.
Radiotherapy has long been used to treat arteriovenous malformations (AVM), a benign condition involving pathological vessel formation, in the brain. An AVM is a congenital vascular pathology characterized by tangles of veins and arteries. The dose applicable to the treatment of neovascularization in age-related macular degeneration (WAMD) by the devices described herein may be based on stereotactic radiosurgery (SRS) treatment of arteriovenous malformations (AVM). SRS is used to deliver radiation to the AVM in order to obliterate it, and radiation is highly effective for AVM treatment. The minimum dose needed to obliterate an AVM with high probability is approximately 20 Gy. However, small AVMs (<1 cm) are often treated with a higher dose (e.g., 30 Gy) because when treating small AVMs, a significant amount of eloquent brain (e.g., brain regions wherein injury typically causes disabling neurological deficits) is not exposed to the high dose of radiation. The reported SRS doses correspond to the dose received at the periphery of the AVM, while the dose at the nidus (center) may be up to a factor of 2.5 times greater than the reported SRS dose.
The vascular region involved in WAMD is much smaller than even the smallest AVM, thus the effective doses are expected to be similar to the highest doses used for AVM. Studies of irradiation of WAMD have shown that greater than Gy are required, although one study indicates some response at 16 Gy. Without wishing to limit the present invention to any theory or mechanism, the devices described herein for WAMD are expected to be effective by delivering a nearly uniform dose to the entire region of neovascularization or by delivering a nonuniform dose which may vary by a factor of 2.5 higher in the center as compared to the boundary of the region with minimum doses of 20 Gy and maximum doses of 75 Gy. A report using radiosurgery for macular degeneration describes that a dose of only Gy was not effective (Haas et al, J Neurosurgery 93, 172-76, 2000). In that study, the stated dose is the peripheral dose with the center being about 10% greater. Furthermore, the study results were severely plagued by retinal complications.
Without wishing to limit the present invention to any theory or mechanism, it is believed that the devices of the present invention are advantageous over the prior art. For example, since SRS employs external photon beams which easily penetrate the ocular structures and pass through the entire brain, the patient must be positioned such that the beams may be directed towards the macula, making the geometric uncertainties of delivery a few millimeters. The devices of the present invention have geometric and dosimetric advantages because they may be placed at the macula with submillimeter accuracy, and the beta radioisotope may be used to construct the radiation source with predominately limited range.
The present invention features methods and devices for minimally-invasive delivery of radiation to the posterior portion of the eye.